The Billikens held Butler scoreless for 4:13 late in the first period while building a 17-point lead and they were never threatened.
Jett scored 19 points, hitting 8-of-12 from the field as part of St. Louis' 50 percent shooting performance. Dwayne Evans added 15 points for the Billikens and Rob Lee had 12.
The outcome further muddled the tight Atlantic 10 Conference race, where seven teams have two losses and no team has fewer.
St. Louis (15-5, 4-2 A-10) forced 23 turnovers while committing only 10.
Rotnei Clarke scored 17 points to lead Butler (17-4, 4-2).